The U.S. Navy is pursuing plans to purchase an advanced fleet of aircraft despite the protests of President Donald Trump.

Lockheed Martin’s F-35 has been described by some as the most costly weapons system in history, and the Navy's plans to put the aircraft into service as part of its Joint Strike Fighter program have prompted a major debate within the defense industry. The Navy's F-35C variant, capable of landing on aircraft carriers, would ultimately replace aging squadrons of Boeing F/A-18 Super Hornets, aircraft that are popular among much of the military's top brass. Putting the F-35s into service would cost the Navy nearly $400 billion, a price tag that prompted the president's criticism.

“The Navy still plans to procure two squadrons of Boeing’s F/A-18 and two squadrons of Lockheed Martin’s F-35C variant, which can take off and land on the deck of an aircraft carrier," Navy Adm. Bill Moran said Tuesday at the Navy League's Sea Air Space symposium, according to Politico.

In December, Trump took to Twitter to describe the F-35 program's costs as "out of control." His tweets, which suggested that the funds would be better used elsewhere in the military, reportedly caused Lockheed Martin's stock to fall by as much as 5 percent. A month later, after Trump had been sworn in, his defense secretary, James Mattis, ordered a comparison of the F-35 and F/A-18s. Around the same time, the F/A-18's manufacturer, Boeing, began circulating a document suggesting that updating the F/A-18 fleet would be more efficient than purchasing the F-35s, and that F-35s would leave the Navy with "significant capability gaps against emerging threats," according to the Navy Times. The series of events raised doubts as to the F-35's future in the military.

Trump has since appeared to have altered his position. After claiming to have cut $600 million off the purchase price of 90 F-35s, Trump told Congress in February that he had "saved taxpayers hundreds of millions of dollars by bringing down the price of the fantastic new F-35 jet fighter," according to Bloomberg News. Trump's remarks came after multiple conversations with Air Force Lieutenant General Christopher Bogdan, who has headed the F-35 program and has fought for the aircraft's implementation. Other officials have said the program would "form the backbone of Navy air combat superiority for decades to come."

"The Navy remains dedicated to a capabilities-focused approach as we evolve the carrier wing and carrier strike group of the future. The dynamic security environment requires the speed, endurance, flexibility and autonomous nature of the carrier strike group," Navy Rear Admiral DeWolfe Miller told Congress in February, according to a statement emailed to Newsweek by the Navy Office of Information.

While the F/A-18s have been considered extremely capable aircraft, their constant use has reportedly resulted in a number of maintenance issues and highlighted the military's need to replace or overhaul the jets, which were first introduced in the 1980s. The F-35 has already begun to see action in Air Force training missions that simulated Russian and Chinese defenses. The Navy's first F-35Cs arrived in 2013, according to the U.S. Naval Institute, and the jets are scheduled to be fully operational in the force by 2018, according to military news outlet Scout.

“One of the key reasons we bought this airplane is because the threats continue to evolve.... This airplane gives us the ability to penetrate, deliver weapons and then share that information across the formation that it is operating in,” Air Force Major Geneneral Jeffrey Harrigian told Scout in an interview published Saturday.

Trump's defense budget seeks to allocate $2.32 billion for both F-35s and F/A-18s, according to the Navy Times, meaning the latter jet is unlikely to be abandoned altogether in the near future. His decision to include the F-35, however, signaled a significant victory for defense officials looking to modernize and expand the U.S. military's capabilities under the new administration.